Rover: fix set_target_location to always use Guided mode

Without this fix, if lua sent a target location while in Guided-within-Auto the Auto submode would be changed out of Guided-within-Auto meaning follow-up commands from lua or a companion computer would be ignored

Rover/Rover.cpp

@@ -165,7 +165,7 @@ bool Rover::set_target_location(const Location& target_loc)
         return false;
     }
 
-    return control_mode->set_desired_location(target_loc);
+    return mode_guided.set_desired_location(target_loc);
 }
 
 // set target velocity (for use by scripting)